Anneal techniques for chalcogenide semiconductors

ABSTRACT

Techniques for precisely controlling the composition of volatile components (such as sulfur (S), selenium (Se), and tin (Sn)) of chalcogenide semiconductors in real-time—during production of the material are provided. In one aspect, a method for forming a chalcogenide semiconductor material includes providing a S source(s) and a Se source(s); heating the S source(s) to form a S-containing vapor; heating the Se source(s) to form a Se-containing vapor; passing a carrier gas first through the S-containing vapor and then through the Se-containing vapor, wherein the S-containing vapor and the Se-containing vapor are transported via the carrier gas to a sample; and contacting the S-containing vapor and the Se-containing vapor with the sample under conditions sufficient to form the chalcogenide semiconductor material. A multi-chamber processing apparatus is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 14/499,116filed on Sep. 27, 2014, the contents of which are incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to chalcogenide semiconductors such askesterite and chalcopyrite materials and more particularly, totechniques for precisely controlling the composition of volatilecomponents (such as sulfur (S), selenium (Se), and tin (Sn)) ofchalcogenide semiconductors via real-time anneal atmosphere compositioncontrol during production of the material.

BACKGROUND OF THE INVENTION

The ratio of sulfur (S) to selenium (Se) in chalcogenide (e.g.,chalcopyrite, kesterite) semiconductors is important for optimal bandgap and/or band gap grading. See, for example, T. K. Todorov et al,“High-Efficiency Solar Cell with Earth-Abundant Liquid-ProcessedAbsorber,” Advanced Materials 22, E156-E159 (February 2010).Device-quality material formation typically occurs at temperatures above450° C. at which S, Se and other key constituents such as zinc (Zn)metal and tin (Sn) chalcogenides are volatile. Furthermore thevolatility of Sn species and Sn depletion of the CZTS can stronglydepend on the chalcogen type and vapor pressure in the anneal atmosphere(see, for example, A. Redinger et al., “The Consequences of KesteriteEquilibria for Efficient Solar Cells,” J. Am. Chem. Soc. 133, 3320-3323(February 2011) (hereinafter “Redinger”) and J. J. Scragg et al.,“Chemical Insights into the Instability of Cu₂ZnSnS₄ Films duringAnnealing,” Chem. Mater., 23(20), pp. 4625-4633 (September 2011)(hereinafter “Scragg”)). Therefore precise real time atmospherecomposition control during fabrication is important in order to achieveoptimal equilibrium reaction conditions and achieve the targeted profileof volatile species in the material.

With conventional processes, atmosphere compositions of S and Se areoften controlled using gases such as hydrogen sulfide (H₂S) or hydrogenselenide (H₂Se). H₂S and H₂Se are however highly toxic. There is aconsiderable expense associated with using and maintaining these gasessafely. There are currently no straightforward methods forchalcogenide-dependent Sn vapor control in the anneal atmosphere.

Thus, techniques for effectively regulating S, Se and (optionally Sn)anneal atmosphere composition during device-grade chalcogenidesemiconductor production without the use of toxic components such as H₂Sand H₂Se would be desirable.

SUMMARY OF THE INVENTION

The present invention provides techniques for precisely controlling thecomposition of volatile components (such as sulfur (S), selenium (Se),and tin (Sn)) of chalcogenide semiconductors via real-time annealatmosphere control—during production of the material. In one aspect ofthe invention, a method for forming a chalcogenide semiconductormaterial is provided. The method includes the steps of: providing atleast one sulfur source and at least one selenium source; heating the atleast one sulfur source to form a sulfur-containing vapor; heating theat least one selenium source to form a selenium-containing vapor;passing a carrier gas first through the sulfur-containing vapor and thenthrough the selenium-containing vapor, wherein the sulfur-containingvapor and the selenium-containing vapor are transported via the carriergas to a sample containing at least one precursor component of thechalcogenide semiconductor material; and contacting thesulfur-containing vapor and the selenium-containing vapor with thesample under conditions sufficient to form the chalcogenidesemiconductor material.

In another aspect of the invention, a multi-chamber processing apparatusis provided. The apparatus includes a sequence of chambers connected inseries such that an outlet of one of the chambers is connected to aninlet of an adjacent one of the chambers in the sequence; at least onesulfur source in the first chamber in the sequence; at least oneselenium source in a second chamber in the sequence; optionally at leastone tin source in a third chamber in the sequence; and a source of acarrier gas connected to the first chamber in the sequence.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary methodology for forming achalcogenide semiconductor material according to an embodiment of thepresent invention;

FIG. 2 is a diagram illustrating an exemplary multi-chamber processingapparatus for forming a chalcogenide semiconductor material according toan embodiment of the present invention;

FIG. 3 is a diagram illustrating an exemplary methodology for forming achalcogenide semiconductor material using the multi-chamber processingapparatus of FIG. 2 according to an embodiment of the present invention;

FIG. 4 is a diagram illustrating an exemplary embodiment forindependently controlling and monitoring the temperature within eachchamber in the multi-chamber processing apparatus of FIG. 2 according toan embodiment of the present invention;

FIG. 5 is a diagram illustrating another exemplary multi-chamberprocessing apparatus for forming a chalcogenide semiconductor materialaccording to an embodiment of the present invention; and

FIG. 6 is a diagram illustrating an exemplary implementation of thepresent techniques to control the sulfur (S) to selenium (Se) ratio insample CZTS kesterite materials according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are techniques for precisely controlling the compositionof the volatile components (such as sulfur (S), selenium (Se), and/ortin (Sn)) of a chalcogenide semiconductor in real-time—during productionof the material. As highlighted above, control over the ratio of S to Sein chalcogen semiconductor materials is an important factor inoptimizing band gap and/or band gap grading. Conventional processeshowever often employ highly toxic gases such as hydrogen sulfide (H₂S)and hydrogen selenide (H₂Se) for sulfurization/selenization purposes.Costly measures must then be put in place to insure that these gases areused and maintained safely. By contrast, the present techniquesadvantageously employ elemental sources of sulfur and selenium.Elemental sulfur and selenium are significantly less toxic than theirsulfide and selenide counterparts. Further, as will be described indetail below, the present techniques involve the production of discreteamounts of sulfur- and selenium-containing vapor that, once no longerneeded, rapidly condense back to a non-gaseous form.

The term “chalcogenide semiconductor material,” as used herein, refersgenerally to any semiconductor material having at least one chalcogen.Generally, elements from group 16 of the periodic table of elements areconsidered as chalcogens. Notably, however, in semiconductor devicetechnology, S and Se are most often the chalcogens of interest.Exemplary chalcogenide semiconductors which can be formed in accordancewith the present techniques include, but are not limited to, materialscontaining copper (Cu), indium (In), gallium (Ga), and at least one of Sand Se (abbreviated herein as “CIGS”) and materials containing Cu, zinc(Zn), Sn and at least one of S and Se (abbreviated herein as “CZTS”).

In general, the present techniques involve passing a carrier gasconsecutively through heated elemental or compound sources starting withthe lowest temperature to the highest temperature thus simplifyingequipment design and increasing reliability of the process control. Anoverview of the present techniques is now presented by way of referenceto methodology 100 of FIG. 1. While methodology 100 will be described inthe context of forming a chalcogenide semiconductor material, it is tobe understood that the present techniques are more broadly applicable toany fabrication process in which precise control of one or more volatilecomponents is desirable.

In step 102, at least one S source and at least one Se source areprovided. As highlighted above, the present techniques will be used tocontrol the S to Se ratio in the chalcogenide semiconductor material.Additionally, in some exemplary embodiments presented herein, othervolatile components of the material are also optionally controlled viathe present process. For instance, kesterite materials also includevolatile components such as Sn. An exemplary kesterite material is onecontaining copper (Cu), zinc (Zn), Sn and at least one of S and Se(CZTS). Controlled amounts of Sn may also be introduced during thepresent process. In that case, in step 102, at least one source of Sn isalso provided. An exemplary multi-chamber processing apparatus will bedescribed below wherein each source material (e.g., S, Se, andoptionally Sn) is located within a separate chamber in sequence based onboiling points of the sources.

According to an exemplary embodiment, the S source is elemental S andthe Se source is elemental Se. The use of elemental sources avoids thetoxic sulfides and selenides commonly employed in other processes.

According to an exemplary embodiment, when present, the Sn source is aSn-containing compound. Regarding chalcogenide materials that contain Snsuch as CZTS, the volatile nature of Sn at temperatures commonlyemployed during processing make the precise composition of the materialdifficult to control. Namely, when kesterite samples are heated above400° C., reevaporation of Sn occurs causing desorption and loss of Snfrom the samples. See, for example, Mitzi et al., “The path towards ahigh-performance solution-processed kesterite solar cell,” Solar EnergyMaterials & Solar Cells 95 (June 2011) 1421-1436, the contents of whichare incorporated by reference as if fully set forth herein. It isnotable that the Sn lost due to desorption is however generally notelemental Sn, but occurs primarily in the form of tin sulfide (SnS).See, for example, A. Weber et al., “On the Sn loss from thin films ofthe material system Cu—Zn—Sn—S in high vacuum,” Journal of AppliedPhysics 107, 013516 (January 2010) (hereinafter “Weber”), the contentsof which are incorporated by reference as if fully set forth herein.Carrying out high temperature anneals (even at temperatures of 550° C.or above) in a sulfur environment can thus mitigate the loss of Sn. SeeWeber. Therefore, it is preferable to use a compound Sn source thatcontains sulfur and/or selenium rather than elemental tin. According toan exemplary embodiment, the Sn source is a solid Sn-containing compoundselected from tin (II) sulfide (SnS), tin (IV) sulfide (SnS₂), tinsulfoselenide (Sn(S/Se)_(x)), and combinations including at least one ofthe forgoing compounds. The toxicity of these low vapor pressure sulfideand selenide materials is less of an issue compared to volatilemetalorganic sources due to quick condensation and easy separation assoon as the temperature of the exhaust gas drops.

Preferably, excess amounts of the (S, Se, and/or Sn) sources areprovided in step 102. By “excess” it is meant that greater amounts ofthe (S, Se, and/or Sn) sources are provided in step 102 than arecontained in a single sample in order to shift the solid-gas equilibriaat elevated temperatures towards desired kesterite/chalcopyrite phaseand composition. Thus, the same sources may be used in multipleiterations of methodology 100 to prepare multiple samples. Further,since the present process permits real-time control over the compositionof each of the source components, the same sources (S, Se, and/or Sn)provided in step 102 can be used in multiple runs to producechalcogenide semiconductor materials having different compositions.

It is notable that in many applications what is important is the amountof one component relative to one or more other components of thematerial. For instance, in the case of a kesterite or chalcopyritematerial, the volatility of certain components makes the composition ofthe material difficult to control. Simply providing excess amounts ofthese volatile components (e.g., amounts of these components in excessof what is needed to satisfy the stoichiometric requirements of the endproduct) can serve to compensate for their loss due to evaporationduring processing. Excess amounts of the volatile components can beremoved from the sample by heating up the sample to a sufficienttemperature (so that these elements have a sufficient vapor pressure)and for a sufficient time to evaporate the excess from the surface ofthe sample. See, for example, U.S. Patent Application Publication Number2013/0037090 filed by Bag et al., entitled “Capping Layers for ImprovedCrystallization,” the contents of which are incorporated by reference asif fully set forth herein. However, beyond simply providing excessvolatile components to compensate for their loss during processing, thepresent techniques advantageously permit adjustment of the ratios of thecomponents relative to one another. For instance, as highlighted above,band gap tuning in kesterite and chalcopyrite semiconductor materials iscarried out by controlling the S to Se ratio in the material. By way ofthe present techniques, precise control over the ratio of volatilecomponents such as S and Se can be easily achieved.

In step 104, the S source is heated to form a S-containing vapor. Aswill be described in detail below, a carrier gas will be passed throughthe S-containing vapor. In step 104, the S source is heated to saturatethe carrier gas with the S vapor at the given temperature (see below).The S concentration in the carrier gas is directly related to thetemperature to which the S source is heated in step 104 (preferably atemperature above its melting point—however some vapor will form evenbelow this temperature) and contact of the S source with the carriergas. Regarding temperature, according to an exemplary embodiment the Ssource is heated to a given temperature of from about 180° C. to about300° C., and ranges therebetween to saturate the carrier gas with theS-containing vapor at the given temperature. Regarding contact of the Ssource with the carrier gas, sufficient contact between the S source andthe carrier gas can be achieved, e.g., by bubbling the carrier gasthrough molten sulfur (see, for example, FIG. 2—described below).Sufficient contact insures that the S concentration at every giventemperature and pressure is constant. According to an exemplaryembodiment, the S concentration in the carrier gas is from about 0.1% toabout 20%, and ranges therebetween. As provided above, the presenttechniques advantageously permit precise regulation of the volatilecomponents used in chalcogenide semiconductor material formation, suchas S, Se, and (optionally) Sn. For instance, the precise S to Se ratioemployed is important for band gap tuning and/or for band gap grading.According to the present techniques, the ratio of S to Se produced invapor form can be easily regulated by regulating the temperature of therespective sources. Thus, according to an exemplary embodiment, in step106 the temperature at which the at least one S source is heated isregulated to thereby regulate an amount of the S-containing vaporproduced.

Having the ability to regulate the amount of S (as well as that of theSe and Sn (if Sn if present)—see below) in this manner permits real-timecontrol of the composition of the chalcogenide semiconductor material,i.e., control over the composition of the material while it is beingformed. For instance, to regulate the S to Se ratio, one simply has toregulate the temperature of the S source relative to the Se source, orvice versa. It is further noted that once the source is permitted tocool, the source will condense (back to its solid or liquid form). Asprovided above, it is preferable to provide excess amounts of each ofthe sources (i.e., amounts of the sources in excess of what is needed totreat a single sample) such that the same sources provided in step 102can be used in multiple iterations of methodology 100 to processmultiple samples (of potentially different composition). In that case,the composition of the chalcogenide semiconductor materials produced ineach iteration can, if desired, be varied in real-time from oneiteration to another by simply (independently) regulating thetemperature of the sources (as per step 106 for S and steps 110 and 114for Se and Sn, respectively—see below).

In step 108, the Se source is heated to form a Se-containing vapor. Aswill be described in detail below, the carrier gas will be passedthrough the Se-containing vapor. In step 108, the Se source is heated tosaturate the carrier gas with the Se vapor at the given temperature (seebelow). The Se concentration in the carrier gas is directly related tothe temperature to which the Se source is heated in step 108 (preferablya temperature above its melting point—however some vapor will form evenbelow this temperature) and contact of the Se source with the carriergas. Regarding temperature, according to an exemplary embodiment the Sesource is heated to a given temperature of from about 300° C. to about450° C., and ranges therebetween to saturate the carrier gas with theSe-containing vapor at the given temperature. Regarding contact of theSe source with the carrier gas, sufficient contact between the Se sourceand the carrier gas can be achieved, e.g., by passing the gas overmolten selenium in an elongated container (see, for example, FIG.2—described below). Sufficient contact insures that the Se concentrationat every given temperature and pressure is constant. According to anexemplary embodiment, the Se concentration in the carrier gas is fromabout 0.1% to about 20%, and ranges therebetween. As provided above, thepresent techniques advantageously permit precise regulation of thevolatile components used in chalcogenide semiconductor materialformation, such as S, Se, and (optionally) Sn. For instance, the preciseS to Se ratio employed is important for band gap tuning and/or for bandgap grading. According to the present techniques, the ratio of S to Seproduced in vapor form can be easily regulated by regulating thetemperature of the respective sources. Thus, according to an exemplaryembodiment, in step 110 the temperature at which the at least one Sesource is heated is regulated to thereby regulate an amount of theSe-containing vapor produced.

Increasing the temperature for either the S source or the Se source willincrease the relative amount of S- or Se-containing vapor, respectively.Experimental data illustrating the effect of source temperaturevariation on S to Se ratio of the component vapor is provided below.With regard to tuning the band gap of a chalcogenide semiconductormaterial, it is notable that pure CZTSe (i.e., 100 percent (%) Se) has aband gap of 0.96 electron volts (eV) and pure CZTS (i.e., 100% S) has aband gap of 1.5 eV. Thus by way of the present techniques the band gapof a chalcogenide semiconductor material can be tuned within that rangeby varying the S to Se ratio.

In step 112, the Sn source (if present) is heated to form aSn-containing vapor. Step 112 is optional since a vapor source of Sn maynot be needed in all applications (such as in the case where thechalcogenide semiconductor material (e.g., CIGS) does not contain Sn).As will be described in detail below, the carrier gas will be passedthrough the Sn-containing vapor (if present). In step 112, the Sn sourceis heated to saturate the carrier gas with the Sn vapor at the giventemperature (see below). The Sn concentration in the carrier gas isdirectly related to the temperature to which the Sn source is heated instep 112 and contact of the Sn source with the carrier gas. Regardingtemperature, according to an exemplary embodiment the Sn source isheated to a given temperature of from about 500° C. to about 800° C.,and ranges therebetween to saturate the carrier gas with theSn-containing vapor at the given temperature. Regarding contact of theSn source with the carrier gas, sufficient contact between the Sn sourceand the carrier gas can be achieved, e.g., by passing the gas over solidtin in an elongated container (see, for example, FIG. 2—describedbelow). Sufficient contact insures that the Sn concentration at everygiven temperature and pressure is constant. According to an exemplaryembodiment, the Sn concentration in the carrier gas is from about 0.1%to about 20%, and ranges therebetween. As provided above, the presenttechniques advantageously permit precise regulation of the volatilecomponents used in chalcogenide semiconductor material formation, suchas S, Se, and in this case Sn. Thus, according to an exemplaryembodiment, in step 114 the temperature at which the at least one Snsource is heated is regulated to thereby regulate an amount of theSn-containing vapor produced. From the above description, it is apparentthat according to the present techniques the temperature profiles of thedifferent sources and therefore the concentrations of volatilecomponents in the carrier gas can optionally be varied independentlyduring the course of the process, for example supplying higherconcentration at higher temperatures of lower vapor pressure componentssuch as Sn in order to produce optimal semiconductor properties.

In step 116, a carrier gas is passed first through the S-containingvapor, then through the Se-containing vapor, and finally (if present)through the Sn-containing vapor. The S-containing vapor, theSe-containing vapor, and the (optional) Sn-containing vapor aretransported via the carrier gas to a sample. Suitable carrier gasesinclude, but are not limited to, helium, nitrogen, and argon gas.

According to an exemplary embodiment, the sample contains at least oneprecursor component of the chalcogenide semiconductor material. Forinstance, as precursor components the sample may contain at least onemetal component of the chalcogenide semiconductor material (such as Cu,Zn, and/or Sn in the case of a CZTS material, or Cu, In, and/or Ga inthe case of a CIGS material). Optionally, the sample may also contain achalcogen component (e.g., S and/or Se) in a stoichiometric amount forthe final composition, or amounts less or more than the stoichiometricamounts. For example, while the present techniques may serve tointroduce S and Se to the sample to reach final composition, this is notthe only scenario. The present techniques may also be applied when thesample has the desired S and/or Se composition and these vapors areintroduced in the anneal atmosphere to prevent S and/or Se loss at thehigh temperatures required for proper crystallization. See, for example,Redinger and Scragg, the contents of both of which are incorporated byreference as if fully set forth herein.

By way of example only, the sample may include the precursorcomponent(s) of the chalcogenide semiconductor material in the form of alayer or multiple layers on a substrate. The term “precursor” refers tothe notion that the final composition and/or distribution of elementsthroughout the final chalcogenide semiconductor material will beestablished only after the sample has been subjected to a finalhigh-temperature crystallization anneal, been contacted with S-, Se-,and/or (optional) Sn-containing vapor, and cooled to roomtemperature—see below. Thus, the elements as they presently exist in thesample are at this stage merely precursors for the final chalcogenidesemiconductor material formation. Even if the precursors present in thesample represent all of the elements present in the final chalcogenidesemiconductor material (for example, the precursors for a CZTS materialmight include each of Cu, Zn, Sn, S, and Se), the final composition ofthe material will not be established until the high-temperatureequilibria between the solid and vapor components are reached andmaintained and/or modified during the temperature ramp down. Further,since the present techniques involve the volatile components of thesematerials (e.g., S, Se, and/or Sn), the source of the non-volatilecomponents of the material (e.g., the above-mentioned metals) need to bealready present in the sample.

It is further noted that the S source has the lowest boiling point ofall of the sources in this example. The Se source has the next lowestboiling point, followed by the Sn source (if present). By passing thecarrier gas first through the S-containing vapor, followed by theSe-containing vapor and finally (if present) the Sn-containing vapor,the component-containing (S-, Se-, and/or Sn-containing) vapor alwaysmoves from a lower to a higher temperature environment. This isimportant, because if the component-containing vapor was to instead movefrom a higher to a lower temperature environment (e.g., one at atemperature below the boiling point of one (or more) of the componentsin the vapor), then the components would condense back to a liquid orsolid and no longer be available in vapor form for reaction with thesample.

Finally, in step 118, the component-containing vapor (e.g., the S-, theSe-, and if present the Sn-containing vapor) is contacted with thesample under conditions (e.g., temperature, duration, etc.) sufficientto form the chalcogenide semiconductor material. According to anexemplary embodiment, the component-containing vapor is contacted withthe sample at a temperature of from about 450° C. to about 650° C., andranges therebetween, e.g., from about 500° C. to about 600° C., andranges therebetween, for a duration of from about 1 second to about 24hours, and ranges therebetween. Preferably, the temperature employed instep 118 is greater than or equal to the temperature needed to maintainthe component having the highest boiling point in the vapor form. Ashighlighted above, this is done to prevent the vapor components fromcondensing. These annealing conditions (temperature and duration) instep 118 are commensurate with a final high-temperature crystallizationanneal, after which the sample(s) is permitted to cool to roomtemperature. As provided above, it is only after the finalhigh-temperature anneal and cool down that the final composition and/ordistribution of elements throughout the final chalcogenide semiconductormaterial will be established.

According to an exemplary embodiment, methodology 100 is carried out ina multi-chamber processing apparatus, such as exemplary multi-chamberprocessing apparatus 200 shown illustrated in FIG. 2. Processingapparatus 200 includes a sequence of chambers 202 (i.e., chambers 202 a,b, c, d, etc.), inlets and outlets of which are interconnected by gaspassages 210 (i.e., gas passages 210 a, b, c, d, etc.). Chambers 202 a,b, c, d, etc. are connected in series such that—as will be described indetail below—the carrier gas (and with it the component-containing(i.e., S-, Se-, and/or Sn-containing) vapor) can readily pass insequence from the first chamber to the last. According to an exemplaryembodiment, each source (e.g., S source, Se source, and (optional) Snsource) is placed in a separate one of the chambers 202. Therefore, thenumber of chambers employed in processing apparatus 200 will depend onthe number of sources and thus on the final composition of thesemiconductor material being produced. Accordingly, the number ofchambers 202 shown in FIG. 2 is merely one exemplary configurationintended solely to illustrate the present techniques, and more or fewerchambers 202 may be employed than is shown in FIG. 2.

A detailed description of the chambers 202 will be provided inconjunction with the description of FIG. 4, below. In general however,each of the chambers 202 is preferably formed from a heat resistantmaterial such as metal, glass or plastic, and is gas-tight in the sensethat the only pathway for gases into and out of the chambers is throughthe inlets and outlets interconnecting the chambers.

Further, the placement of the sources in the chambers 202 will be basedon the boiling point of the sources such that during methodology 100 thecomponent-containing vapor is always transported via the carrier gasfrom a lower temperature environment to a higher temperature environment(in this case form a lower temperature chamber to a higher temperaturechamber). As described above, this insures that the components in thevapor do not condense, but rather remain in vapor form during theprocess. To use an illustrative example, an S source would be located ina chamber earlier in the sequence than an Se source since S has a lowerboiling point than Se. Likewise, both the S source and the Se sourcewould be located in chambers earlier in the sequence than a Sn sourcesince both S and Se have lower boiling points than Sn, and so on. In theexample shown illustrated in FIG. 2, the first three chambers 202 a, 202b, and 202 c contain sources 212, 214, and 216, respectively, while thefourth chamber (the last chamber in the sequences) contains the sample218. During operation, the temperature of each of the chambers 202 isindependently regulated. Each chamber 202 a, 202 b, 202 c, and 202 d isthus considered herein to be a separate temperature zone (i.e.,temperature zone 1, temperature zone 2, temperature zone 3, andtemperature zone 4, respectively). See FIG. 2.

An exemplary embodiment wherein methodology 100 is performed usingapparatus 200 is now described by way of reference to methodology 300 ofFIG. 3. The process begins in step 302 with the multi-chamber processingapparatus 200. As described above, multi-chamber processing apparatus200 includes a sequence of chambers 202 (i.e., chambers 202 a, b, c, d,etc.) connected in series such that an outlet of each chamber isconnected to an inlet of an adjacent chamber in the sequence.

In step 304, at least one S source is placed in the first chamber in thesequence and at least one Se source is placed in the second chamber inthe sequence. In the exemplary processing apparatus 200 shown in FIG. 2,chamber 202 a is the first chamber in the sequence and chamber 202 b isthe second chamber in the sequence. The order of the sequence isdictated by the direction of the flow of the carrier gas through thechambers. Arrows are used in FIG. 2 to indicate the flow of the carriergas. As provided above, the ordering of the sources in the sequence ofchambers is relevant to placing the sources in order from lowest tohighest boiling point along the direction of flow of the carrier gas.

According to an exemplary embodiment, at least one Sn source is alsopresent and in step 304 is placed in the third chamber in the sequence.Since not all implementations include Sn (such as is the case when, forexample, a CIGS chalcogenide semiconductor material is being formed) aSn source is optional. However, when present, the Sn source is placed ina chamber after both the S source and the Se source in the sequence ofchambers since Sn has a higher boiling point than both S and Se. Thus,based on the direction of flow of the carrier gas through the chambers(see arrows in FIG. 2), the vapor from each source will be carried tosuccessively higher temperature environments so as to insure that eachcomponent-containing vapor is kept at or above the boiling point of itsconstituent components. If the component-containing vapor was, on theother hand, transported to a chamber having a temperature below theboiling point of one or more of the vaporized components then thosecomponents would undesirably condense before reaching the sample.

As described above, suitable S and Se sources include elemental S andelemental Se, respectively. In the exemplary embodiment shownillustrated in FIG. 2, elemental S and Se sources have been heated intheir respective chambers (i.e., chambers 202 a and 202 b) until molten.Thus, the S and Se sources are shown in FIG. 2 to be liquid (molten)sources. FIG. 2 further illustrates that when liquid sources are presentit is possible to introduce the carrier gas through the liquid (i.e.,the carrier gas is bubbled through the liquid source). Bubbling thecarrier gas through the liquid source increases contact of the carriergas with the component-containing vapor. This exemplary configuration isshown being implemented in (elongated) chamber 202 a of apparatus 200wherein the carrier gas is being delivered via a tube 206 to below thesurface of the liquid (molten) S source 212. The carrier gas thenbubbles up through the molten S source 212. Alternatively, it is alsopossible to simply pass the carrier gas over the heated liquid source.This exemplary configuration is shown being implemented in chamber 202 bof apparatus 200 wherein the carrier gas is simply passed over theliquid (molten) Se source 214. This example also highlights the notionthat the chambers 202 of apparatus 200 do not have to have the sameshape as one another. For instance, when bubbling the carrier gasthrough a liquid source it may be desirable to configure the chamber tobe, like chamber 202 a, narrow yet deeper than the other chambers whichwill provide (based on the path of the carrier gas through the chamber)a greater vertical contact area between the carrier gas and thecomponent-containing vapor. Conversely, when the carrier gas is simplybeing passed over the surface of a liquid source it may be desirable toconfigure the chamber to be, like (elongated) chamber 202 b, wider yetshallower than chamber 202 a which will provide (based on the path ofthe carrier gas through the chamber) a greater horizontal contact areabetween the carrier gas and the component-containing vapor. Thedepiction of different approaches for passing the carrier gas eitherthrough or over a liquid source in FIG. 2 is meant merely to illustratedifferent possible techniques. It is not intended to imply that theliquid S source and the liquid Se source necessarily need to beprocessed differently.

In FIG. 2, the Sn source 216 is shown to be a solid. As provided above,suitable Sn sources include, but are not limited to, Sn-containingcompounds such as SnS, SnS₂, and/or Sn(S/Se)_(x). By way of exampleonly, the Sn source 216 can consist of a layer of the Sn-containingcompound that has been formed on a suitable substrate material (e.g., ona glass plate). In that case, it may also be desirable to employ anelongated chamber design that, like chamber 202 b, is wide and shallowwhich will provide (based on the path of the carrier gas through thechamber) a greater horizontal contact area between the carrier gas andthe component-containing vapor.

As shown in FIG. 2 the sample 218 is placed in the last chamber in thesequence of chambers. In this example, the last chamber is the fourthchamber 202 d. As provided above, the sample contains at least oneprecursor component of the final chalcogenide semiconductor material,for example, at least one metal and/or at least one chalcogenidecomponent of the chalcogenide semiconductor material. Chalcogenidesemiconductor materials are often employed as absorber layer materialsin photovoltaic devices. Thus, by way of example only, the sample mayinclude at least one of the precursors for a photovoltaic deviceabsorber layer, and formation of the absorber layer can be carried outin accordance with the present techniques. Further, it is noted thatmultiple samples may be treated at the same time and in the same mannerdescribed. Thus, the illustration of a single sample 218 in FIG. 2 ismerely an example.

In step 306, the at least one S source 212 is heated to form aS-containing vapor in the first chamber 202 a. The process for heatingan elemental S source to produce a S-containing vapor was described indetail above. As also described above, the amount by which the S sourceis heated is directly proportional to the amount of the S-containingvapor produced. Thus, in step 308, the temperature at which the at leastone S source 212 is heated is regulated to thereby regulate the amountof the S-containing vapor produced in chamber 202 a. As will bedescribed in detail, for example, in conjunction with the description ofFIG. 4 below, the temperature in each chamber 202 of apparatus 200 canbe independently regulated, thereby permitting independent regulation ofthe amount of the component-containing vapor produced in each chamber.

In step 310, the at least one Se source 214 is heated to form aSe-containing vapor in the second chamber 202 b. The process for heatingan elemental Se source to produce a Se-containing vapor was described indetail above. As also described above, the amount by which the Se sourceis heated is directly proportional to the amount of the Se-containingvapor produced. Thus, in step 312, the temperature at which the at leastone Se source 214 is heated is independently regulated to therebyregulate the amount of the Se-containing vapor produced in chamber 202b.

In step 314, the at least one Sn source 214 (if present) is heated toform a Sn-containing vapor in the third chamber 202 c. The process forheating a compound Sn source to produce a Sn-containing vapor wasdescribed in detail above. As also described above, the amount by whichthe Sn source is heated is directly proportional to the amount of theSn-containing vapor produced. Thus, in step 316, the temperature atwhich the at least one Sn source 216 is heated is regulated to therebyindependently regulate the amount of the Sn-containing vapor produced inchamber 202 c.

In step 318, the carrier gas is then passed through the sequence ofchambers beginning with the first chamber 202 a. As described above, thesources are arranged in the chambers by their boiling points. Therefore,in this example, the first chamber 202 a (containing the S source) islocated before the second chamber 202 b (containing the Se source) inthe sequence of chambers, and the second chamber 202 b is located beforethe third chamber 202 c (containing the Sn source if present) in thesequence of chambers such that the carrier gas passes first through theS-containing vapor in the first chamber 202 a, then through theSe-containing vapor in the second chamber 202 b, and finally through theSn-containing vapor (if present) in the third chamber 202 c. The carriergas transports the S-containing vapor, the Se-containing vapor and (ifpresent) the Sn-containing vapor through the sequence of chambers to thesample.

As shown in FIG. 2, a source of the carrier gas is connected to thefirst chamber 202 a. By way of example only, the carrier gas source maybe a gas cylinder or tank. The gas cylinder or tank may be fitted with apressure regulator to regulate the pressure of the carrier gas beingintroduced into the first chamber 202 a of apparatus 200. The pressureshould be high enough that the carrier gas flows in only onedirection—from the first chamber to the last. According to an exemplaryembodiment, the carrier gas is introduced into the first chamber at apressure of from about 5 pounds per square inch gauge (psig) to about100 psig, and ranges therebetween.

As highlighted above, a stream of the carrier gas will pass through eachof the chambers 202 in succession picking up the component-containingvapor from each chamber it passes through. More specifically, as thecarrier gas passes through the first chamber 202 a it passes through theS-containing vapor present in the first chamber 202 a. Since thechambers are connected in series, the carrier gas then passes from thefirst chamber 202 a into the second chamber 202 b (via gas passage 210a) transporting the S-containing vapor from the first chamber to thesecond chamber. As the carrier gas passes through the second chamber 202b it passes through the Se-containing vapor present in the secondchamber 202 b. The carrier gas stream then passes from the secondchamber 202 b into the third chamber 202 c via gas passage 210 b. As thecarrier gas stream passes through the third chamber 202 c it passesthrough the Sn-containing vapor present in the third chamber 202 c. Thecarrier gas stream then passes from the third chamber 202 c into thefourth chamber 202 d via gas passage 210 c. In this example, the fourthchamber contains the sample 218. As noted above, the particular numberof chambers 202 present in apparatus 200 is based on the number ofsources employed and thus on the particular chalcogenide semiconductormaterial being produced. For instance, in the case of a CIGS material, achamber to accommodate a Sn source would not be needed.

Finally, in step 320, the component-containing vapor (e.g., the S-, theSe-, and if present the Sn-containing vapor) is contacted with thesample 218 in the fourth chamber 202 d under conditions (e.g.,temperature, duration, etc.) sufficient to form the chalcogenidesemiconductor material. According to an exemplary embodiment, thecomponent-containing vapor is contacted with the sample 218 at atemperature of from about 450° C. to about 650° C., and rangestherebetween, e.g., from about 500° C. to about 600° C., and rangestherebetween, for a duration of from about 1 second to about 24 hours,and ranges therebetween (which represent conditions commensurate with afinal high-temperature crystallization anneal). Preferably, thetemperature employed in step 320 is greater than or equal to thetemperature needed to maintain the component having the highest boilingpoint in the vapor form. As highlighted above, this is done to preventthe vapor components from condensing.

Specifically, as the carrier gas stream passes through the fourthchamber 202 d it contacts the sample 218 contained in the fourth chamber202 d. The component-containing (S-, Se-, and/or Sn-containing) vaportransported by the carrier gas reacts with the sample to form the endproduct chalcogenide semiconductor material. The carrier gas thentransports the component-containing vapor out of the fourth chamber 202d and the apparatus 200 via gas passage 210 d. As shown in FIG. 2, afilter may be employed to remove the vapor components from the carriergas stream as it exits the last chamber of the apparatus. Membrane orelectrostatic filters or other purification systems known in the art mayalso be employed, for example scrubbers.

As provided above, the temperature of each chamber 202 in apparatus 200can be independently regulated to regulate the temperature at which thesource contained in the chamber is heated and thereby control the amountof component-containing vapor produced in the chamber. Thus, accordingto the present techniques, the temperature profiles of the differentsources and therefore the concentrations of volatile components in thecarrier gas can optionally be varied independently during the course ofthe process, for example supplying higher concentration at highertemperatures of lower vapor pressure components such as Sn in order toproduce optimal semiconductor properties. Independent control over thetemperature of each of the chambers 202 in apparatus 200 may be achievedusing a number of different heating mechanisms. For example, in itssimplest form, a separate means of heating is provided for each of thechambers 202. In one exemplary embodiment, each of the chambers isequipped with its own (independently controlled) resistive heatingelement. See for example FIG. 4 which shows a representativeimplementation of separate resistive heating elements applied to each ofthe chambers 202. For illustrative purposes only, FIG. 4 depictschambers 202 b and 202 c of apparatus 200 from FIG. 2 with theunderstanding that the same configuration may be employed in any of theother chambers. In the exemplary configuration shown illustrated in FIG.4, each chamber 202 consists of a (e.g., metal, glass or plastic)enclosure 402 fitted with a gas tight removable lid 404. The lid 404permits a source material or sample to be easily placed in/removed fromthe chamber. The lid 404 is gas tight to prevent the carrier gas or thecomponent-containing vapor from leaving the chamber other than by way ofthe gas passages 210.

In this example, a separate resistive heating element 406 is locatedwithin each of the chambers 202. Resistive heating elements may beconstructed from a strip of electrically conductive material arranged,e.g., as a coil. The ends of the resistive heating element pass from theinside to the outside of the enclosure 402 where they are connected to acontroller 408. In order to independently control the temperature ofeach resistive heating element, each resistive heating element isconnected to a separate controller 408 a, b, etc. (labeled “Controller1,” “Controller 2,” etc.). In its simplest form, each controller 408 maybe an adjustable power supply. As shown in FIG. 4, a seal is used aroundthe ends of the resistive heating element as they pass through the wallof the enclosure 402 which prevents gases from leaking out of thechamber 202. By way of example only, the seal used can be an adhesiveaffixing and sealing the ends of the resistive heating element to theenclosure.

Preferably, a means for monitoring the temperature in each chamber 202is also provided. By way of example only, a separate thermocouple may beused in each chamber. See FIG. 4. As is known in the art, a thermocoupleis a temperature measuring device that produces a voltage in response tochanges in temperature. Depending on thermocouple type (e.g., type K,type C, type R, etc.) an appropriate material combination can beselected which will be resilient to the needed temperature changes andprovide reliable readings at the desired temperatures. A suitablethermocouple device for the present apparatus includes leads 410 whichpass though the enclosure to monitor the temperature within eachchamber. See FIG. 4. As described above, a seal (e.g., an adhesive) ispreferably employed to prevent gases escaping from the chamber where theleads 410 pass through the wall of the enclosure.

The gas passages 210 are also configured to prevent gases from escapingat the junction of the gas passages 210 and the inlets/outlets of thechambers 202. For instance in the exemplary embodiment shown illustratedin FIG. 4, each gas passage is formed from a (e.g., metal, glass orplastic) tubing that it sealed at each of its opposing ends to therespective chamber via a flange. The flange can be secured to the wallof the enclosure using mechanical fasteners (such as screws) or anadhesive.

As described above, the number of chambers 202 employed in apparatus 200is based on the number of sources present and thus on the composition ofthe desired chalcogenide semiconductor material. While clearly stated inthe above description that the chamber 202 c shown in FIG. 2 to containthe Sn source 216 might not be needed in all cases, and is thusoptional, for completeness a variation of apparatus 200 is shown in FIG.5 (a multi-chamber processing apparatus 500) that includes a sequence ofthree chambers (502 a, b, and c) connected in series and containing a Ssource 512, a Se source 514, and a sample 516, respectively. Thetemperature of each chamber may be independently regulated in the samemanner described above. Except here, there are only 3 temperature zones.Inlets and outlets of the chambers are connected via gas passages 510(i.e., gas passage 510 a, b, and c). Methodology 100 of FIG. 1 may becarried out in multi-chamber processing apparatus 500 in the same manneras described above, except that only two sources (in this case S and Se)may be used. Thus, for instance, the Sn source would be absent. By wayof example only, apparatus 200 is well suited for forming CZTS kesteritematerials having three volatile components S, Se, and Sn, whereasapparatus 500 is well suited for forming CIGS chalcopyrite materialshaving two volatile components S and Se.

FIG. 6 is a diagram illustrating an exemplary implementation of thepresent techniques to control the S to Se ratio in sample CZTS kesteritematerials. Specifically, the compositions of two CZTS samples A and Bare shown. The samples were processed in accordance with the presenttechniques during which the S and Se source temperatures were varied. Asshown in FIG. 6, an increase in the S source temperature of from 210° C.to 220° C. and a decrease in the Se source temperature of from 330° C.to 320° C. between sample A and sample B resulted in an increase in theS to Se ratio of the material. For instance, the S to Se composition ofthe sample A is closer to an almost pure Se kesterite (96% Se) while theS to Se composition of the sample B is closer to a pure S kesterite(100% S).

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A multi-chamber processing apparatus, comprising:a sequence of chambers connected in series via gas passages such that anoutlet of one of the chambers is connected via a gas passage to an inletof an adjacent one of the chambers in the sequence and wherein, for atleast a given one of the chambers, the inlet and the outlet are separatefrom one another and are located at opposite ends of the given chamberwith at least one source being present within the given chamber betweenthe inlet and the outlet; at least one sulfur source in a first chamberin the sequence; at least one selenium source in a second chamber in thesequence; and a source of a carrier gas connected to the first chamberin the sequence, wherein the carrier gas is passable through thesequence of chambers in series via the gas passages; at least one tinsource in a third chamber in the sequence; heating elements within eachof the chambers, wherein the heating elements comprise resistive heatingelements, and wherein a separate resistive heating element is locatedwith each of the chambers; and controllers connected to each of theheating elements, wherein each of the controllers comprises anadjustable power supply; wherein the carrier gas is selected from thegroup consisting of: helium gas, nitrogen gas, and argon gas wherein theat least one tin source comprises a tin-containing compound selectedfrom the group consisting of: tin (II) sulfide, tin (IV) sulfide, tinsulfoselenide, and combinations comprising at least one of the forgoingcompounds; and a filter at the gas passage out of a last chamber in thesequence, wherein the filter is configured to remove vapor componentsfrom a stream of the carrier gas as the stream of the carrier gas exitsthe last chamber in the sequence.
 2. The multi-chamber processingapparatus of claim 1, further comprising a sample in a last chamber inthe sequence, wherein the sample contains at least one precursorcomponent of a chalcogenide semiconductor material.
 3. The multi-chamberprocessing apparatus of claim 2, wherein the chalcogenide semiconductormaterial comprises a kesterite material, and wherein the at least oneprecursor component of the chalcogenide semiconductor material isselected from the group consisting of: copper, zinc, tin, sulfur,selenium, and combinations comprising at least one of the foregoingmetals.
 4. The multi-chamber processing apparatus of claim 2, whereinthe chalcogenide semiconductor material comprises a chalcopyritematerial, and wherein the at least one precursor component of thechalcogenide semiconductor material is selected from the groupconsisting of: copper, indium, gallium, sulfur, selenium, andcombinations comprising at least one of the foregoing metals.
 5. Themulti-chamber processing apparatus of claim 1, wherein the at least onesulfur source comprises elemental sulfur.
 6. The multi-chamberprocessing apparatus of claim 1, wherein the at least one seleniumsource comprises elemental selenium.
 7. The multi-chamber processingapparatus of claim 1, wherein each of the chambers is formed from amaterial selected from the group consisting of: metal, glass andplastic.
 8. The multi-chamber processing apparatus of claim 1, whereineach of the chambers is gas-tight such that an only pathway for gasesinto and out of the chambers is through the gas passages interconnectingthe chambers.
 9. The multi-chamber processing apparatus of claim 1,wherein the source of the carrier gas is a gas cylinder fitted with apressure regulator to regulate a pressure of the carrier gas beingintroduced into the first chamber.
 10. The multi-chamber processingapparatus of claim 1, wherein the source of the carrier gas is connectedvia a tube to the first chamber in the sequence, wherein a first end ofthe tube is connected to the source of the carrier gas and a second endof the tube is present below a surface of the at least one sulfursource.
 11. The multi-chamber processing apparatus of claim 1, whereinat least one of the chambers has a different shape from at least oneother of the chambers in the sequence.
 12. The multi-chamber processingapparatus of claim 1, wherein the at least one tin source comprises alayer of the tin-containing compound disposed on a substrate.
 13. Themulti-chamber processing apparatus of claim 12, wherein the substratecomprises a glass plate.